1. Field of the Invention
The present invention relates to a driving technique for a light emitting element, and particularly to an overcurrent protection technique for such an element.
2. Description of the Related Art
In recent years, as a backlight of a liquid crystal panel or as an illumination device, a light emitting apparatus is employed, which is configured using a light emitting element such as an LED (light emitting diode) or the like. FIG. 1 is a circuit diagram which shows an example configuration of a light emitting apparatus according to a comparison technique. A light emitting apparatus 1003 includes an LED string 6, and a switching power supply 1004.
The LED string 6 includes multiple LEDs connected in series. The switching power supply 1004 is configured to step up an input voltage Vin input to an input terminal P1 so as to output a driving voltage Vout to one terminal of the LED string 6 connected to an output terminal P2.
The switching power supply 1004 includes an output circuit 102 and a control IC 1100. The output circuit 102 includes an inductor L1, a switching transistor M1, a rectifier diode D1, and an output capacitor C1. The control IC 1100 adjusts the driving voltage Vout by controlling the on/off duty ratio of the switching transistor M1.
A PWM dimming switch (transistor) M2 and a current detection resistor R1 are arranged on a path of the LED string 6. A controller 1010 generates a PWM dimming pulse signal G2 having a duty ratio that is controlled according to the target luminance level. A driver DR2 performs switching of the PWM dimming switch M2 according to the pulse signal G2.
A voltage drop (detection voltage) VR1, which is proportional to the driving current IDRV that flows through the LED string 6, occurs at the detection resistor R2. An error amplifier EA1 amplifies the difference between the detection voltage VR1 and the control voltage VREF so as to generate a feedback voltage VFB. The controller 1010 generates a gate pulse signal G1 that is subjected to pulse modulation according to the feedback voltage VFB. A driver DR1 performs switching of the switching transistor M1 according to the gate pulse signal G1.
With such a configuration, feedback control is performed such that the relation IDRV=VREF/R2 holds true, thereby allowing the LED string 6 to emit light with a luminance level that corresponds to the control voltage VREF.